Rockwool Sound insulation into rafters and party walls

13 Jul 2019
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United Kingdom

I have 80mm gap between my rafters and would like to use the Rockwool sound insulation, as this is the 100mm I am planning on cutting it down to 30mm (i've been told I can) and place it between the rafter.

I'm not to sure about both of my neighbours walls, I would like some insulation there but some sound deadening too. I have already made a frame using 2x4s, can I use the same sound insulation or would I have to do anything different?

Edit. Also, if I'm using 30mm Rockwool between the rafters I assume I should be using something thinner outside (on top) of the rafters. If so, can i use a 50mm between the rafters and 30mm outside? or even 40mm between the rafters and 25/30mm outside.
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I don't understand. Acoustic insulation is a poor choice for insulating a roof. You can buy it in whatever thickness you need. The thickness of insulation fitted between rafters is usually determined by the ventilation requirements of the roof timbers with the overall thickness (including under) by the thermal performance required.
What is the better choice for a roof? Is this insulation any good for the walls?
PIR (celotex) is the best for thermal insulation (reducing heat loss) in the roof. Thermal insulation in the party wall is (was) not generally considered necessary if next door is occupied (in my case my neighbours are pensioners who heat at full blast 24/7 so we probably gain into our cold house). The rock wool has good performance both thermally and acoustically for walls - if you need it - if not then 100mm is a fait bit of space to loose.
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I meant rafters not joists.


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I want to place insualtion in the ceiling and the walls to have the house warmer. I would also like to reduce the noise too while i'm at it. The loft will be used for storage to begin with but the kids will eventually take over as a play room

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